PluralEyes is strictly a pre-editing tool. If you have an edited sequence and are trying to sync it with PluralEyes, it likely will not work.
That said, there is a workaround with Final Cut Pro 7 wherein you can replace the camera audio of an edit with another source (presumably a higher-quality external audio source).
- Create a new project and make a sequence containing your unedited footage
- Sync that sequence using the sync options Replace Audio and Single Sequence Output
- This will create a set of merged clips in the Browser
- Make a QuickTime reference movie for each merged clip (right-click on the clip and choose Export > QuickTime Movie...)
- Open the project with your edited sequence (or to be safe, make a copy of the project and open that)
- For each video clip in the Browser, reconnect the media to the QuickTime movies made in Step 4
- Your project will now have the good audio and the edits will be preserved